1. Field of the Disclosure
Embodiments of the present disclosure generally relate to drill bits for drilling earth formations. More specifically, embodiments of the present disclosure relate to the geometry of cuttings elements of roller cone drill bits. More specifically still, embodiments of the present disclosure relate to geometries of gage insert cuttings elements of roller cone drill bits.
2. Background Art
Traditionally, drilling systems used to drill earth formation include a drilling rig that is used to turn a drill string, which extends downward into a wellbore. Connected to the end of the drill string is a roller cone drill bit. Disposed on the drill bit are a plurality of cutting elements used to break away pieces of the formation during drilling.
In roller cone bits, the cutting elements drill the earth formation by a combination of compressive fracturing and shearing action. Prior art milled tooth bits typically have teeth formed from steel or other easily machinable high-strength material, to which a hardface overlay such as tungsten carbide or other wear resistant material is often applied. The hardfacing is applied by any one of a number of well known methods, There are a number of references which describe specialized exterior surface shapes for the substrate.
Specialized shapes are intended to provide a cutting structure which includes more thickness of hardface overlay in wear-prone areas, so that the useful life of the teeth may be increased. Examples of such specialized substrate shapes are shown in U.S. Pat. Nos. 5,791,423, 5,351,771, 5,351,769, and 5,152,194. These references show that the teeth have a substantially regular trapezoidal exterior hardface surface. The irregular shape of the substrate outer surface is selected to provide additional hardface in the wear prone areas while maintaining a conventional exterior tooth surface.
U.S. Pat. No. 6,029,759 issued to Sue shows a milled tooth drill bit having teeth in a gage row (i.e., the outermost row of teeth on any cone used to maintain full drilling diameter), wherein the teeth have a particular outer surface. The particular outer surface of these teeth is intended to make it easier to apply hardfacing in two layers, using two different materials. The purpose of such tooth structures is to have selected hardfacing materials positioned to correspond to the level of expected wear on the various positions about the outer surface of the tooth.
Polycrystalline diamond (“PCD”) enhanced inserts and tungsten carbide (“WC-Co”) inserts are two commonly used inserts for roller cone rock bits and hammer bits. A roller cone rock bit typically includes a bit body adapted to be coupled to a rotatable drilling string and include at least one cone that is rotatably mounted to the bit body. The cone typically has a plurality of inserts pressed into the surface. The inserts thus contact the formation during drilling.
The PCD layer on PCD enhanced inserts is extremely hard. As a results, the PCD layer has excellent wear resistance properties. While the actual hardness of the PCD layer varies for the inserts used in particular bit types, each type of PCD has a common failure mode of chipping and spalling due to cyclical impart loading on the inserts during drilling. Conversely, the softer, tougher tungsten carbide inserts tend to fail by excessive wear and not by chipping and spalling.
Examples of tungsten carbide inserts used on the gage row of roller cone bits include relieved gage chisel inserts and semi-round top inserts. Relieved gage chisel inserts are manufactured by increasing carbide on the leading side of the hole wall surface of the cutting element and increasing relief on the trailing side of the hole wall surface. Such relieved gage chisel inserts were designed for soft formation drill bits where the compressive forces are lower relative to harder formation. A second insert, the semi-round top insert is used in the gage row of hard formation drill bits. Because of the symmetric nature of the dome shaped cuttings portion of the insert, the insert may eventually break due to trailing side chipping after gage rounding, which may thereby result in additional insert breakage and/or drill bit failure.
When the gage row of a drill bit begins to fail due to, for example, chipped trailing edges of individual gage inserts, there is an increased likelihood of the entire drill bit failing. If a drill bit fails, the entire drill string must be removed from the wellbore, section-by-section, a process referred to as “tripping.” Because the drill string may be miles long, tripping the drill string requires considerable time, effort, and expense. As such it is desirable to employ drill bits that are less prone to gage row failure that may ultimately result in a costly trip of the drill string.
Accordingly, there exists a need for hard formation cutting elements for roller cone drill bits that are more resistant to wear and chipping during drilling.